1. Field of the Invention
The present invention relates to a lithographic projection apparatus.
2. Related Art
Lithographic projection apparatus are used in the manufacture of integrated circuits (ICs), flat panel displays (FPDs), and other devices involving fine structures. An array of individually controllable elements generates a pattern corresponding to an individual layer of, for example, the IC, and this pattern is imaged onto a target portion (e.g., comprising part of one or several dies) on a substrate (e.g., a silicon wafer, glass plate, or the like) that has been coated with a layer of radiation-sensitive material (e.g., resist).
Prior to this imaging step, the substrate may undergo various procedures, such as priming, resist coating, and a soft bake. After exposure, the substrate may be subjected to other procedures, such as a post-exposure bake (PEB), development, a hard bake, and measurement/inspection of the imaged features. These transfer procedures result in a patterned layer of resist on the substrate. One or more pattern steps, such as deposition, etching, ion-implantation (doping), metallization, oxidation, chemo-mechanical polishing, etc., follow, each of which is intended to finish, create or modify a layer of the device. If several layers are required, then the whole procedure, or a variant thereof, is repeated for each new layer. Eventually, an array of devices will be present on the substrate. These devices are then separated from one another by a technique such as dicing or sawing, whence the individual devices can be mounted on a carrier, connected to pins, etc. Further information regarding such processes can be obtained, for example, from the book “Microchip Fabrication: A Practical Guide to Semiconductor Processing”, Third Edition, by Peter van Zant, McGraw Hill Publishing Co., 1997, ISBN 0-07-067250-4, which is incorporated herein by reference.
In many cases, the packing density of the individually controllable elements within an array of individually controllable elements may be relatively low (i.e., the active area of each element that can modulate the beam of radiation is a relatively small fraction of the total area of the element). Therefore a significant proportion of the radiation within the radiation beam cannot be modulated. In other words, this radiation is either reflected by the array of individually controllable elements or absorbed by the array of individually controllable elements, regardless of the pattern set on the array.
It has previously been proposed to include a micro lens array within the radiation system in order to provide a projection beam that is made up of a plurality of spots; and then to project this beam onto the active areas of the array of individually controllable elements. However, projecting such a projection beam of radiation onto the array of individually controllable elements and subsequently projecting the patterned beam onto the substrate requires a complex and consequently expensive projection system.
Therefore, what is needed is a way of illuminating only active areas of an array of individually controllable elements without requiring a complex projection system.